1. Field of the Invention
The present invention relates to a variable focal distance lens system and an imaging device. More in detail, the present invention particularly relates to a technical field of a variable focal distance lens system and an imaging device used in a video camera, a digital still camera, and so forth, and having an angle of view exceeding seventy-five degrees in a wide-angle end state and a zoom ratio exceeding ten times.
2. Description of the Related Art
In the past, a method has been used which employs, as recording means in a camera, an image pickup device using photoelectric conversion elements such as CCDs (Charge Coupled Devices) and CMOSs (Complementary Metal-Oxide Semiconductors) to record a subject image formed on a surface of the image pickup device by converting the light amount of the subject image into an electrical output with the use of the photoelectric conversion elements.
Along with the advancement of microfabrication technology in recent years, the speed of central processing units (CPUs) and the integration density of storage media have been increased. As a result, a large volume of image data unable to be handled in the past has started to be processed at high speed. Further, light-receiving elements have also been increased in integration density and reduced in size. The increase in integration density has enabled higher spatial frequencies to be recorded, and the reduction in size has enabled the entire camera to be reduced in size.
However, there is an issue in that the above-described increase in integration density and the reduction in size result in a reduction of the light-receiving area of the individual photoelectric conversion elements, and that the resultant decrease in the electrical output is accompanied by an increase in the influence of noise. In view of this, to reduce such influence of noise, there is a configuration in which the aperture ratio of an optical system is increased to increase the amount of light reaching light-receiving elements. Further, there is also a configuration in which minute lens elements called micro-lens arrays are disposed immediately before the respective elements.
The micro-lens arrays guide light fluxes reaching between adjacent elements onto the elements, but limit the exit pupil position of a lens system. This is because the closer to the light-receiving elements the exit pupil position of the lens system is, the larger angle is formed between a principal ray reaching the light-receiving elements and the optical axis, and thus an off-axis light flux directed to a screen peripheral area forms a large angle with respect to the optical axis, to thereby prevent a desired amount of light from reaching the light-receiving elements and cause a shortage of the light amount.
In recent years, along with the spread of digital cameras, users' requirements have become diverse.
Particularly, a camera small in size but including a zoom lens having a high magnification ratio (a variable focal distance lens system) is desired, and zoom lenses having a magnification ratio exceeding ten times are provided.
In general, a positive-negative-positive-positive four-group type has been used as a type representing the configuration of a zoom lens having a high magnification ratio.
The positive-negative-positive-positive four-group type zoom lens is configured to include a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power, which are sequentially disposed from the object side to the image side. In the positive-negative-positive-positive four-group type zoom lens, during a change in positional state of lenses from a wide-angle end state with the shortest focal distance to a telescopic end state with the longest focal distance, each of the first to third lens groups moves such that the interval between the first and second lens groups is increased and the interval between the second and third lens groups is reduced, and the fluctuation of the image plane position is compensated for by the movement of the fourth lens group.
As such a positive-negative-positive-positive four-group type zoom lens, the zoom lens described in Japanese Unexamined Patent Application Publication No. 2008-146016, for example, is used.
Further, in recent years, wide-angle zoom lenses having an angle of view exceeding seventy-five degrees have increased. As one of such wide-angle zoom lenses, a so-called negative-lead zoom lens including a first lens group having negative refractive power has been often used in the past.
For example, the zoom lens described in Japanese Unexamined Patent Application Publication No. 2007-94174 is configured to include a first lens group having negative refractive power and a second lens group having positive refractive power, which are sequentially disposed from the object side to the image side.
Further, the zoom lens described in Japanese Unexamined Patent Application Publication No. 2008-46208 is configured to include a first lens group having negative refractive power, a second lens group having positive refractive power, a third lens group having negative refractive power, and a fourth lens group having positive refractive power, which are sequentially disposed from the object side to the image side.
Further, in recent years, aspherical lenses have been commonly used, and a so-called positive-lead zoom lens including a first lens group having positive refractive power has also been often used.
Such a positive-lead zoom lens includes the zoom lenses described in Japanese Unexamined Patent Application Publication Nos. 2008-102165, 2007-72117, and 2008-203453, and so forth.
For example, in the positive-lead zoom lens described in Japanese Unexamined Patent Application Publication No. 2008-102165, many aspherical lenses are used to achieve a wide angle and a high magnification, and the third lens group is configured to include a positive lens having two convex surfaces, and a meniscus-shaped negative lens having a concave surface facing the image side, which are sequentially disposed from the object side to the image side.
In the zoom lens described in Japanese Unexamined Patent Application Publication No. 2007-72117, the third lens group is configured to include a cemented lens formed by a positive lens having a convex surface facing the object side and a negative lens having a concave surface facing the image side, and a positive lens, which are sequentially disposed from the object side to the image side.
In the zoom lens described in Japanese Unexamined Patent Application Publication No. 2008-203453, the third lens group is configured to include a positive lens having two convex surfaces, and a cemented lens having negative refractive power and formed by a positive lens having a convex surface facing the object side and a negative lens having a concave surface facing the image side, which are sequentially disposed from the object side to the image side.